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OSPF routers "R1" and "R2" are not forming an adjacency. Which of the following could cause the problem? (choose three)

The Router-ID was not configured on each router.

Both routers are configured in different areas.

Each router has different costs in the directly connected link.

An ACL is configured on the routing protocol.

The Process ID's are configured differently on each router.

A passive interface is configured on a router.

Explanation

For two routers to form an OSPF adjacency, they must be in the same area, must be in the same subnet, and must have the same authentication (if configured). If an ACL or a passive interface is set, it might avoid OSPF updates to be transmitted or received.